Conventionally, a technique is known where an actual air-fuel ratio is detected by an air-fuel ratio sensor or an oxygen concentration sensor mounted on an exhaust system and a failure determination based on the actual air-fuel ratio is carried out in a fuel injection system which feedback-controls the air-fuel ratio of an engine. That is, in the above technique, it is determined that any kind of failure occurs in the fuel injection system in a case where the actual air-fuel ratio greatly deviates from a target air-fuel ratio even when feedback control is performed with an aim to adjust the actual air-fuel ratio such that the ratio becomes close to the target air-fuel ratio.
Meanwhile, in the above technique, a sudden change in the fuel injection amount, the actual air-fuel ratio, the target air-fuel ratio, or the like due to a change in a fuel infection mode causes the actual air-fuel ratio to temporarily fluctuate greatly, which may lead to an erroneous determination. Therefore, it is proposed that a failure determination based on the actual air-fuel ratio is prohibited during a period from when the fuel injection mode is changed until a fixed time passes in order to prevent an erroneous failure determination (For example, see JP 4640012 B1).
However, if a failure determination is prohibited without exception when the fuel injection mode is changed, discovery of a failure which occurs in the fixed time period will be naturally delayed. Therefore, failure diagnosis accuracy lowers. In addition, delay in discovery of the failure causes the air-fuel ratio to be kept inappropriately controlled in an unstable engine combustion state, which may lower stability and reliability of engine control. Such a problem becomes more evident especially as engine rotation speed is higher.